Tapered slot antennas have been in use extensively as linear polarized radiators. In most applications, linearly tapered slot antennas or exponentially tapered slot antennas, commonly known as notch antennas or Vivaldi antennas, are used. Linear slot antennas have been disclosed in U.S. Pat. No. 4,855,749 (DeFonzo); exponentially tapered slot antennas have been disclosed in U.S. Pat. No. 5,036,335 (Jairam) and U.S. Pat. No. 5,519,408 (Schnetzer). In particular, DeFonzo discloses the design of an opto-electronic tapered slot transceiver, made on a silicon on sapphire substrate wherein the slotline can be linearly or exponentially tapered. Jairam discloses an improved balun for electromagnetically coupling the slotline with a feedline in a Vivaldi antenna. The return loss of the improved balun significantly out performs that of a conventional feed in which a straight length of the slotline is coupled to a straight length of a feedline at right angles, separated by a dielectric layer. The conventional Vivaldi antenna with conventional feed is shown in FIG. 1. As shown in FIG. 1, the Vivaldi antenna 2 is an exponentially tapered slot formed on a dielectric substrate 4, defined by two opposite members 6, 7 of a metallized layer 5 on one side of the substrate. The feedline 1 is a narrow conductor located on the other side of the substrate, crossing over the extended portion 3 of the slotline at right angles, forming a balun D. For comparison, the return loss patterns of an exponentially tapered slot antenna with conventional feed (dotted line) and that with Jairam's improved feed (solid line) are shown in FIG. 2. Schnetzer discloses a Vivaldi slot antenna fed by a section of a slotline and a coplanar waveguide. Schnetzer also discloses an array of Vivaldi antennas being incorporated on a thin substrate having thereon a copper conductor layer and each antenna is fed from a coplanar waveguide feed network. The major disadvantage of the Vivaldi configuration is that the return loss performance does not meet the requirements of today's broadband communication applications.
In recent years, there has been a tremendous demand on broadband antenna arrays to be used in cellular telephones or communication devices operated in PCS frequencies. Other applications such as interferometer array for direction finding and early warning RADAR also require broadband operations. Thus, it is advantageous to provide a coplanar antenna array with broadband capability for operations over multiple frequency bandwidths.